Single-molecule (SM) measurements have already yielded many insights into photophysical processes and are being increasingly used to study all types of molecular systems from simple dye molecules to fluorescent proteins.[1–3] Electron transfer (ET) has not received much attention at the SM level as, once it is operative in a molecule, the fluorescence is usually heavily quenched leaving no signal to be detected. Lu and Xie [4] made the first SM study of ET using cresyl violet at an indium–tin oxide (ITO) interface and Xie and co-workers [5, 6] have also studied ET from tyrosine to flavin residues in proteins. Adams and co-workers [7] have studied interfacial ET between a perylene di-imide and an ITO interface by SM techniques and, recently, Lu and co-workers [8] studied interfacial ET in dye–TiO2 nanoparticles with either Coumarin343 or Cresyl Violet as the dye. Work on molecular donor–acceptor systems is even scarcer, with only a few examples to date. Liu and coworkers [9] studied ET in a perylene di-imide (PI) dimer and we studied ET at the SM molecule in a dendrimer containing 16 peripheral triphenylamine (TPA) electron donors and a central perylene di-imide (PDI) acceptor [10] by exploiting the phenomenon of delayed fluorescence. We also studied the conformational dynamics [11] and the role of oxygen [12] in the ET processes for this dendrimer and an analogue containing eight TPA chromophores.

One current issue in SM spectroscopy is the origin of extended periods (tens of milliseconds and longer) of no emission, so-called “long off-times”. Shorter off-times due to triplet blinking are well understood and usually last on the order of microseconds to a few milliseconds.[13] Longer off-times have proven difficult to study due to their infrequency and “dark” nature. By studying small ensembles of Rhodamine 6G mole-